Everything you ever wanted to know about "How did Kepler arrive at his second law?" but were afraid to ask ;) is contained in a talk entitled "Astronomical eggs, laid and unlaid, in Keplerian planetary theories ..." delivered by D.T. Whiteside to the BSHM on December 15, 1972. You're warmly driven to take a look at the _Journal for the History of Astronomy_ 5 (1974), pp. 1-21.